The invention concerns a system arrangement for the drive train of lifting mechanisms, in particular crane lifting mechanisms, comprising at least one drive motor, at least one cable drum connected thereto, a reduction transmission arranged between the drive motor and the cable drum, an automatic overrun shutdown means, and at least one safety brake and a method of operating the system arrangement.
In a known lifting mechanism of the specified kind (EP 1 661 845 B1) there are provided two drive motors which drive two cable drums by way of a reduction transmission. Provided in the drive train, besides operating brakes and safety brakes, are overrun shutdown means which, in the event of an overload which exceeds a predetermined load, entirely or partially separate the connection between the motors and the cable drums. That is intended to ensure that the individual components of the drive train and more specifically in particular the reduction transmission are neither damaged nor ruined.
In addition a drive train for lifting mechanisms is known (DE 10 2013 209 361 A1) in which, in the case of an emergency stop braking action, damage is avoided by the provision of an automatic overrun shutdown means between the drive motor and the operating brake. The shutdown means is preferably in the form of a freewheel, wherein the freewheel represents an effective safety device if the load to be carried is lowered.
The known systems have already proven their worth in practice. The operating brakes and the safety brakes in the known drive trains are in the form of spring-closing brakes which open hydraulically, pneumatically, magnetically or electro-hydraulically. In the event of a power failure or an emergency shutdown, that has the result that the braking circuits are automatically closed. In that case each braking circuit in itself is capable of stopping the load within the predetermined parameters. The arrangement of the independent braking circuits is substantially due to the fact that on the one hand, in the event of transmission breakdown the load can no longer be stopped with the operating brakes, but on the other hand the operating brakes are required in order to hold the load in the normal case at the rotary speed ‘0’ of the drive motors, with the linked high switching cycles. In accordance with the status at the present time the safety brakes are not suitable for high switching cycles and consequently close only in the event of transmission breakdown, power failure, emergency shutdown and the like.
However a number of problems arise due to the two braking circuits which are implemented in an emergency situation. Due to the shorter dead time firstly the safety brakes operates. In that case the masses building up due to the mass inertias of the motors and motor couplings also have to be braked. High load peaks therefore occur in the reduction transmission. In the load direction ‘LOWER’ the situation additionally involves load changes or tooth flank changes at the gears of the reduction transmission. Those problems can lead to serious transmission damage, in particular in the case of crane lifting mechanisms with particularly frequent shutdown situations and involving high lifting speeds. In addition, due to the operation of both braking circuits, inevitable ‘over-brakings’ of the lifting mechanism occur, with the resultant negative effects on the statics and other crane components.